We’ve discussed HSAs a time or three. As a way to pay for medical expenses, they’re an almost joke.
As a form of tax-advantaged savings, you can think of them as an auxiliary mini-401k. If you’re already deferring the legal max into your 401k ($24.5K plus various catch-ups if you’re old enough), then putting more money into the HSA is a fine idea. As a retirement savings vehicle you intend to invest for the long term. Not as a piggy bank for doctor or pharmacy visits.
The HSA contribution limit is $4400 for a single person and $8750 for a 2+ person family, plus catchups if you’re old enough. So assuming a single person too young for catchups (< age 50), you can top up your 401k’s 24.5K with an extra 4400. An extra ~17%.
When you’re using an HSA as a retirement savings vehicle, then the annual timing you asked about becomes almost immaterial. What matters is to absolutely positively fill it with the full $4400 before yearend so you don’t waste the right to this year’s contribution. Same as an IRA: contributions not made in any year are lost forever.
As a general matter, every savings / investment vehicle performs better the sooner you can fund it. If you could fill your 401K, HSA, and IRA to the legal max on Jan 1, that’d be better than waiting until Dec 31 to do the same thing.
My own take, when I was eligible for an HSA, was to fill my 401K ASAP, then direct the same high saving rate into the HSA until it was full.
Related note:
Some HSAs make it easy to invest most or all of your account value in real investments like stocks and bonds and mutual funds. Other HSAs are pretty much bank savings accounts paying effectively zero interest. It all depends on what provider your employer signed up with and which plan they’re using. Whatever they chose, that’s what you’re stuck with.
An HSA that is a mere savings account with no (or nearly no) investment potential is all but useless as a retirement savings vehicle / auxiliary 401k.
If you’re stuck with one of those, you’d need to work some actual numbers to decide whether the hassle of fiddling with the HSA as just a medical expense piggy bank is worth it. Yes, a dollar saved is a dollar earned. But at some point you’re jumping through hoops for $100/year in savings. Why?